Magnetic pick-up



July 6, 1937. E, EDON 2,086,415

MAGNETIC PICK-UP Filed Dec. 25, 1933 I 0llIIIIIIIIIIIIIIIIIIJIIfl Efizde Guedon,

I maL Patented July 6, 193? MAGNETIC men-Ur Emile (lharles Guedon, Audubon, N. J., a'ssignor to Radio Corporation of America, a corporation of Delaware Application December 23, 1933, Serial No. 703,742

8 Claims.

My invention relates to electro-magnetic translating devices, and more particularly to electrical pick-ups.

p The main object of my invention is to provide an electrical pick-up which can be produced cheaply and which, at the same time, shall have satisfactory electrical and mechanical characteristics. I

A further object of my invention is to provide a pick-up unit that shall be easy toassemble in quantity production.

A further object of my invention is to provide an electrical pick-up having minimum number of parts.

A still further object of my invention is to provide an improved pick-up unit and supporting arm assembly.

In practicing my invention I employ a unitary pole piece construction in a pick-up of the type having a fixed coil threaded by magnetic flux which varies in accordance with the movement of an armature. In this unitary construction, the two pole pieces are integral parts of a member of magnetic material, and are separated by an air gap which is not adjustable. A portion of this member forms part of the support for the armature to hold it in proper relation to the air 1 A pick-up arm is provided which is shaped at one end to form a casing for the pick-up unit, this casing, together with a bottom plate, serving bothto enclose the pick-up unit and to hold its parts in assembled relationship.

Other objects, features and advantages of my 35 invention will appear from the following description taken in connection with the accompanying drawing in which, V

Figure 1 is a view, taken in longitudinal section, of an electrical pick-up constructed in ac- .cordance with my invention;

Fig. 2 is a-view taken on the line 2-2 of Fig. 1; Fig. 3 is a view taken on the line 3-3 of Fig. 1; Fig. 4 is a perspective view 01 a unitary pole piece member comprising one portion of the 45 structure shown in Fig. 1;

Fig. 5 is a bottom view of the pole piece unit shown in Fig. 4;

Fig. 61s a perspective view of a portion of the structure shown in Fig.1: and 5 Fig. 7 is a view taken on the line 1-1 of Fig, 1. Referring to Figs. 1, 2 and 3, the electrical pick-up comprises a pick-up arm I pivotally mounted at one end and shaped at the other end to form a casing 3. The pick-up unit proper 55 consists 0: an assembled unit, indicated generally at 5, and a permanent magnet I. The magnet i is clamped between the pick-up arm casing 3 and the assembled unit '5 as shown in Fig. 3.

The assembled unit 5 comprises an armature 9 having an armature pivot shaft H which extends on opposite sides of the armature. The upper end of the armature 9 is flattened and positioned between pole pieces I 3 and 15. The lower end of the armature 9 is hollow to permit the insertion of a stylus i! which may be clamped in position by means of a set-screw I9 extending into a hollow portion of the pivot shaft I I.

In accordance with my invention, the pole pieces l3 and i5 are part of a unitary construction, the design of which will be more clearly understood by referring to Figs. 4 and 5. This structure consists of a U-shaped member 2| of magnetic material. The upper part of the member 2| consists of two pole piece portions I3 and I5 which are separated by an air gap 23, the adjacent ends of the pole piece portions" being of reduced cross-sectional area as indicated on the drawing.

Directly below the air gap 23 there is an opening 25 in the bottom part 26 of the U-shape member 2| through which the armature 9 may extend. It will he noted that this bottom part functions as a supporting member for the pole piece portions i3 and I5. 0n the under side of the bottom part 26 there is a semi-circular groove 21 located directly underneath the air gap 23 and extending parallel therewith to form an upper bearing surface for the pivot shaft ll of the armature. Tapped holes 29 and 3| are provided on opposite sides of the opening 25 to permit the insertion of screws for clamping the pivot shaft i I between the pole piece structure and a bottom plate 32 described hereinafter.

' A slot 33 is cut from the outer edge 01 the part 26 to the opening 25 in order to prevent the part 26 from acting as a short circuited turn and thereby reducing the efliciency of the pick-up unit and impairing the quality of its output.

The above-mentioned bottom plate 32, which forms the lower supporting member for thepivot shaft H of armature 9 is shown in detail in Fig. 6. It comprises the fiat plate 32, of non-magnetic material, having a groove 35 on the upper side thereof to form a bearing surface for the underside of the pivot shaft l I. -An opening 31 is provided in the middle of the groove 35 to permit the extension of the lower end of the armature 9 therethrough for holding the stylus I1.

At one end of the plate 32 there is an upwardly extending portion 39 having a plurality of tapped holes 48 therein for fastening bottom plate 32 to the pick-up casing 3. In the middle of this portion there is a rectangular opening 33 which is provided to house a rectangular block of inert damping material such as the composition commerciallyknown asViscoloid, or the like if desired. As the use of such damping material is no part of my present invention, it has not been shown in the drawing. It is to be understood, however, that the block is operatively connected to the pivot shaft H in a commercial embodiment of my invention.

- At the other end of the plate 32, there are two upwardly extending portions 35 and 41, the portion 45 being provided to act as a support for a terminal board 59 (Figs. 1 and 2) and the other portion l'l being provided to act as an anchor for one end of a centering spring 5i (Figs/1, 2, and 3).

Again referring to Figs. 1, 2 and 3, it will be seen that the pivot shaft ll of the armature 9 is provided with soft rubber sleeves 53 and 55 and is clamped between the groove supporting surfaces 27 and 35 of the unitary pole piece structure and the bottom plate, respectively. These two members are clamped together by means of two screws 51 and 59 which extend into the tapped holes 3| and 29, respectively, of the pole piece structure.

The rubber sleeves 53 and 55 are provided mainly to reduce friction, but they also act to damp the armature and to center it in the air gap to a certain extent.

An electric pick-up coil 6| in the form of a flat torus is positioned inside the U-shaped pole piece structure and surrounding the armature 9. It will be understood that the coil 6| is slipped into position before the armature 9 is placed in position in the air gap 23.

The pick-up coil 6| is centered with respect to the armature 9 and held in that position by means of a bushing 63 of insulating material which fits tightly in the opening 25 and slightly above the armature pivot shaft II at one end and against the inside of the pole pieces at the other end.

The armature is centered by means of the centering spring 5i which has a sharp bend therein, one end of the spring being soldered or otherwise fastened to one end of the armature pivot shaft H, as by extending through an opening therein. The other end of the spring 5| is positioned between the bottom plate portion 4! and a rectangular washer 65 whereby it may be clamped in portion 65 of the bottom plate by means of a jecting elements 'll'at the top of the casing 3.

The ends of the pick-up coil 6| are next con nected to thetwo conductors 13 of a shielded cable I5 which leads to a suitable amplifier (not shown).

The pickup-arm is supported from a suitable base member "H, a pivot arm l9, and a hollow pin 8|. The pivot arm l9 consists of a horizontal section 83 and a vertical section 85, the horizontal section being pivotally fastened to the sides of the pick-up arm I by means of a. horizontal pin 81 extending through the horizontal section 83 and the said sides. The vertical section 85' has a vertical hole therein, this section being positioned above the base member 11 and held in pivotal relation thereto by means of the pin 8| which is flared out at the bottom. A set-screw 99 holds the pivot arm 19, the pin 8|, and the base )5 extends upwardly from the washer 99 to magnetic material is cut to the desired size and bent into the form of a U. Portions are then removed from the upper side of the bent plate by means of a circular cutting tool to form a comparatively narrow bridge of material in the middle of this side.

The upperside is then formed into two pole pieces separated by an air gap bysawing through the narrow bridge of material. At the same time, a cut is made in the lower side of the bent plate for preventing the short-circuited turn effect. I

Next, the bent plate is turned over and an opening drilled opposite the air gap. Also, the two threaded holes are made on opposite sides of the opening. The groove for the armature pivot shaft is then cut in the plate directly opposite th two outs made by the saw.

It will be noted that the groove is cut so deepthat the plate is thin at the bottom of the groove whereby the flux leakagethrough the metal bridgingthe magnet poles is kept at a minimum value. It will also be noted that the cut made at the back of the U by the circular tool is not deep enough to cut entirely through the curved section joining the flat sides of the U. Thus, the

section indicated at |01- (Fig. 2), together with I the thin plate section at the bottom of the groove, joins the two end portions of the U-shaped unit and holds them in fixed and rigid relationship.

In addition to the previously mentioned advantages of my unitary pole piece construction there is the advantage that the air gap is always uniform since it is sawed or milled out with reference to the semi-circular groove. Another advantage is that the two upper faces of the pole pieces are always exactly in the same plane whereby they make good contact with the pole faces of the permanent magnet to keep the reluctance of the magnetic circuit at a minimum.

Various modifications may be made in my invention without departing from the spirit and 1. In a pick-up unit, a supporting member, a-

pair of pole pieces positioned above said member and having an air gap therebetween, said pole pieces being integral with said supporting member, and a permanent magnet positioned with its pole faces above said pole pieces and having its pole faces directly in contact therewith.

2. In a pick-up unit, a supporting member having a groove cut transversely thereof, a pair of pole pieces attached to said member and positioned above it, said pole pieces having an air gap therebetween which is opposite said groove, a magnet for said pole pieces, a shaft having an armature mounted thereon substantially at right angles thereto, said armature being positioned in said air gap and said shaft being positioned in said groove, and means for holding said shaft in position in said groove.

3. A pick-up unit according to claim 2 char acterized in that said pole pieces are integral with said. supporting member.

4. A pick-up unit according to claim 2 characterized in that said groove is on the under side of said supporting member, and further characterized in that said armature extends through an opening in said member and into said air gap.

5. In a pick-up unit, a supporting member, a pair of pole pieces attached to said member and positioned above it, said pole pieces having an air gap therebetween, a magnet for said pole pieces,

a shaft, an armature mounted thereon substantially at right angles thereto, said armature being positioned in said air gap and said shaft being positioned below said member, and means including a bottom plate for clamping said shaft against said member, said bottom plate forming a portion of a casing forsaid pick-up unit.

6. A unitary pole piece structure for a permanent magnet in an electrical pick-up, said structure comprising a bottom plate and a pair of pole pieces positioned above and spaced away from said bottom plate, said pole pieces and said bottom plate being an integral unit.

7. A unitary pole piece structure for an electrical pick-up, said structure comprising a bottom plate, a pair of pole pieces positioned above and spaced away from said bottom plate, the ends of said pole pieces being separated by an air gap, a connecting section extending from one edge of said bottom plate to a side of one pole piece, and a second connecting section extending from said one edge to a side of the other pole piece.

8. In an electrical pick-up including a fieldmagnet, a vibratory armature having an active end and a stylus carrying end, a translating coil for said armature, an integral pole piece structure for said magnet having portions thereof providing a fixed and nonadiustable air gap for the active armature end, and another portion of said pole piece structure having an opening therein through which the stylus end of said armature extends.

EMILE CHARLES GUEDON. 

